The present invention relates to a choke arrangement in accordance with the preamble of independent claim 1, and to a method in conjunction with an output choke of an inverter in accordance with the preamble of independent claim 5.
An inverter is a device typically used for controlling motors in an economical and reliable manner. At its output, such an inverter generates a voltage or a current with a controllable frequency, enabling a motor to be controlled in an optimal manner in accordance with a desired frequency of the output.
A frequency converter equipped with a voltage intermediate circuit, one being shown in FIG. 7 for the sake of example, generates at its output by means of an inversion part 73 voltage pulses which are short with respect to the basic frequency and whose duration is adjusted to achieve a desired output voltage. Such a frequency converter with a voltage intermediate circuit is provided with a direct voltage intermediate circuit Udc whose positive and negative voltages are coupled as voltage pulses to phased outputs as desired. One phase is thus provided with an upper 71 and a lower 72 semiconductor switch, and the output voltage of the phase is obtained from between the switches. FIG. 7 further shows a motor M controlled by a frequency converter, and an input transformer 74.
These short and fast voltage pulses are generated by power semiconductors resistant to high currents and voltages. Nowadays IGB (insulated gate bipolar) transistors are commonly used for their good properties. An IGBT is capable of cutting off a current of even hundreds of amperes, the maximum voltage of a component being thousands of volts, correspondingly.
In high-current inverters, however, IGBT components have to be coupled in parallel to achieve the necessary current strength. IGBT components are often packed into modules such that one module comprises several IGBT switches and their zero diodes. A natural alternative of parallel coupling the switches of the output of an inverter is then to couple some switches of one module in parallel. Applications that require parallel coupling of several components are typically three-phase ones. Each phase then comprises components coupled in parallel, and these parts of parallel coupling are generally called branches. One phase of an output of an inverter is then composed of a number of branches corresponding to the number of upper and lower switch components coupled in parallel.
In voltage intermediate circuit frequency converters, an output voltage is generated in an inversion part from voltage pulses which, when using existing switch components, have extremely high rates of rise. In a cable between an inverter and a rotatable machine, high rates of voltage rise cause voltage oscillations which strain the insulating materials of the machine. In addition to the rate of voltage rise, the amplitude and frequency of voltage oscillations are determined by the length of such a cable and wave impedance, the wave impedance of the machine as well as other electrical interfaces between the inverter and the machine. The minimum length of a cable to enable maximum reflection to take place in a terminal of a machine is called the critical length of the cable.
A choke at an output of an inverter enables the above-disclosed critical length of a cable to be increased. The desired rate of voltage rise determines the magnitude of the inductance of the choke. The magnitude of the necessary inductance can be reduced by an RC circuit, as shown in FIG. 2. However, using an RC circuit increases losses of drive and may hinder the operation of the control of some inverters.
Conventionally, the rate of voltage rise has thus been restricted by chokes installed at an output of an inverter, which are either single-or three-phase ones. FIG. 1 shows the basic structure of both a single-phase and a three-phase choke. A choke comprises a winding 1 wound around a core 2. The core is usually rectangular in shape and comprises at least one air gap to prevent saturation. In some cases, different voltage cutters are also used for reducing maximum voltages, e.g. as shown in FIG. 3. Here, a diode bridge 31 is coupled to an output of an inverter to cut the voltage of the output when it becomes higher than a voltage Udc of an intermediate circuit.
A high rate of voltage rise also increases a circulating current passing via bearings. It can be roughly said that the circulating bearing current is proportional to the common-mode voltage of a motor and to the resulting current passing through distributed capacitances. This current may be reduced by reducing the rate of voltage rise of the output voltage of an inverter. In addition to an output choke L of an inverter, a special bearing current filter, one being shown in FIG. 8, comprises a common-mode choke Lcomm e.g. in a direct voltage intermediate circuit or at the output of the inverter as well as a capacitance CE installed between a star point of an RC circuit and a ground plane.
Irrespective of whether or not an inverter is equipped with IGBTs in parallel, the purpose of an output choke is to reduce the rate of voltage rise and the amplitude to such a low level that voltage oscillations are incapable of damaging the insulating materials of a rotatable machine and that the current passing via a bearing does not cause any damage to the bearing.
In the known solutions, in conjunction with IGB transistors coupled in parallel, the outputs of the IGB transistors are directly coupled in parallel to form a phase output, a choke being connected to the common current path of this parallel coupling to restrict the rate of voltage rise. In order to prevent saturation of the core of the choke and, on the other hand, to obtain the necessary inductance, the cross-sectional area of the core has to be sufficiently large. Since the current allowed by the components coupled parallel is also high, the necessary choke thus ends up being very large and heavy.
A problem with the direct parallel coupling of IGBT switches is presented by differences between the currents of parallel coupled branches that occur in normal switching situations. The current differences between branches are caused by differences between the mutual conductances and gate capacitances of IGBT switches as well as factors due to a gate driver, such as time diversity of gate control signals and differences between gate voltages and gate resistances. Due to the non-ideality of switch components coupled in parallel and-the circuits controlling such components, an output current of a phase is distributed unevenly between the components during switching situations.
For instance, when switches are being switched off and when one switch starts cutting off the current before the rest of the switches in the same phase, some of the current transfers to other switch components of the particular phase. It has been noted that this increase of current with respect to the dimensioned switch-specific phase current is anything up to dozens of percents. This potential increase of current should be taken into account when designing an inverter. The differences between branch currents reduce the maximum capacity of the inverter because the output current has to be restricted to a level which does not allow the temporary current strength of switch components to be exceeded, not even in the branch with the highest current. The switches thus cannot, be dimensioned on the basis of a phase current only, since such dimensioning could result in a damaged switch component, caused by currents transferring in switching situations.
Another problem in connection with the existing, rigidly parallel coupled branches is that an erroneously operating single-branch power switch, such as an IGBT switch, cannot be identified.